Maria J. Sabater

Supramolecular self-assembled molecules as organic directing agent for synthesis of zeolites.

Solid materials with uniform micropores, such as zeolites, can act as selective catalysts and adsorbents for molecular mixtures by separating those molecules small enough to enter their pores while leaving the larger molecules behind. Zeolite A is a microporous material with a high void volume. Despite its widespread industrial use in, for example, molecular separations and in detergency, its capability as a petroleum-refining material is limited owing to its poor acid-catalytic activity and hydrothermal stability, and its low hydrophobicity. These characteristics are ultimately a consequence of the low framework Si/Al ratio (normally around one) and the resulting high cationic fraction within the pores and cavities. Researchers have modified the properties of type-A zeolites by increasing the Si/Al compositions up to a ratio of three. Here we describe the synthesis of zeolite A structures exhibiting high Si/Al ratios up to infinity (pure silica). We synthesize these materials, named ITQ-29, using a supramolecular organic structure-directing agent obtained by the self-assembly, through π–π type interactions, of two identical organic cationic moieties. The highly hydrophobic pure-silica zeolite A can be used for hydrocarbon separations that avoid oligomerization reactions, whereas materials with high Si/Al ratios give excellent shape-selective cracking additives for increasing propylene yield in fluid catalytic cracking operations. We have also extended the use of our supramolecular structure-directing agents to the synthesis of a range of other zeolites.

A Bifunctional Pd/MgO Solid Catalyst for the One‐Pot Selective N‐Monoalkylation of Amines with Alcohols

It has been found that a bifunctional metal Pd/base (MgO) catalyst performs the selective monoalkylation of amines with alcohols. The reaction goes through a series of consecutive steps in a cascade mode that involves: 1) the abstraction of hydrogen from the alcohol that produces the metal hydride and the carbonyl compound; 2) condensation of the carbonyl with the amine to give an imine, and 3) hydrogenation of the imine with the surface hydrogen atoms from the metal hydride. Based on isotopic and spectroscopic studies and on the rate of each elementary step, a global reaction mechanism has been proposed. The controlling step of the process is the hydride transfer from the metal to the imine. By changing the crystallite size of the Pd, it is demonstrated that this is a structure-sensitive reaction, whereas the competing processes that lead to subproducts are not. On these bases, a highly selective catalyst has been obtained with Pd crystallite size below 2.5 nm in diameter. The high efficiency of the catalytic system has allowed us to extend the process to the one-pot synthesis of piperazines.

On the activity of chiral chromium salen complexes covalently bound to solid silicates for the enantioselective epoxide ring opening

Abstract Two series of solid catalysts in which a chiral chromium salen complex has been anchored on aminopropyl-functionalized SiO 2 , ITQ-2 or MCM-41 have been prepared. In those catalysts in which anchoring of the complex was accomplished through coordination with the metal high enantiomeric excesses were obtained (up to 70% enantiomeric excess (e.e.)), but the complex leaches to the solution on a large extent. In contrast, for those other solids in which the complex is attached to the surface by covalent linkage to the ligand no leaching was observed, but the e.e. were modest (below 20%) compared to those obtained in homogeneous catalysis under comparable conditions (above 50%).

Assessment of the negative factors responsible for the decrease in the enantioselectivity for the ring opening of epoxides catalyzed by chiral supported Cr(III)-salen complexes

In order to determine the influence of the inorganic support on the asymmetric induction, different chiral chromium(III)-salen complexes have been incorporated within the cavities of zeolites Y, EMT and into the interlamellar region of K-10 montmorillonite. These heterogeneous catalysts are able to promote the asymmetric ring opening of epoxides with trimethylsilylazide to afford chiral azido trimethylsilyl ethers and azido alcohols with modest enantiomeric excess that varies depending on the inorganic support. The factors that have been found to play a negative influence diminishing the enantioselectivity of the supported Cr(III)-salen catalyst compared to the unsupported complexes are the following: (i) the presence of adventitious acid sites, (ii) the encapsulation of no sufficiently stereogenic ligands and (iii) the change in the reaction mechanism from bimetallic to a single metal reaction mechanism.

CHIRAL SALEN MANGANESE COMPLEX ENCAPSULATED WITHIN ZEOLITE Y : A HETEROGENEOUS ENANTIOSELECTIVE CATALYST FOR THE EPOXIDATION OF ALKENES

A chiral (salen)Mn III complex [salen: trans-(R,R)-1,2-bis(salicylideneamino)cyclohexane] analogous to Jacobsen’s catalyst is prepared inside the supercages of zeolite Y, showing catalytic activity very similar to that of the chloride complex in the homogeneous phase.

Bifunctional Acid–Base Ionic Liquid Organocatalysts with a Controlled Distance Between Acid and Base Sites

Bifunctional acid-base ionic liquid organocatalysts with different distances between the two sites have been synthesised, and their activity for the Knoevenagel condensation has been tested. As has been found to be the case with enzymes, the distance between the acidic and basic sites determines the activity of the bifunctional organocatalyst, and at the optimal distance the reaction rate increases by two orders of magnitude with respect to the purely acidic or basic counterpart organocatalysts. The experimental results have been rationalised through the study of the reaction mechanism of the Knoevenagel condensation between malononitrile and benzaldehyde by means of DFT calculations. It has been found that it consists of two consecutive steps. First, deprotonation of malononitrile on the basic site to obtain a methylene carbanion intermediate takes place, and second, co-adsorption and activation of benzaldehyde on the acid centre of this intermediate followed by the C-C bond-formation reaction. The calculations and the kinetic study indicate that there is an inversion of the rate-controlling step when the distance between the acidic and the basic sites is modified, with a direct implication on the reaction rate.

Advances in One-Pot Synthesis through Borrowing Hydrogen Catalysis

The borrowing hydrogen (BH) principle, also called hydrogen auto-transfer, is a powerful approach which combines transfer hydrogenation (avoiding the direct use of molecular hydrogen) with one or more intermediate reactions to synthesize more complex molecules without the need for tedious separation or isolation processes. The strategy which usually relies on three steps, (i) dehydrogenation, (ii) intermediate reaction, and (iii) hydrogenation, is an excellent and well-recognized process from the synthetic, economic, and environmental point of view. In this context, the objective of the present review is to give a global overview on the topic starting from those contributions published prior to the emergence of the BH concept to the most recent and current research under the term of BH catalysis. Two main subareas of the topic (homogeneous and heterogeneous catalysis) have been identified, from which three subheadings based on the source of the electrophile (alkanes, alcohols, and amines) have been considered. Then the type of bond being formed (carbon-carbon and carbon heteroatom) has been taken into account to end-up with the intermediate reaction working in tandem with the metal-catalyzed hydrogenation/dehydrogenation step. The review has been completed with the more recent advances in asymmetric catalysis using the BH strategy.

Coupling of Two Multistep Catalytic Cycles for the One‐Pot Synthesis of Propargylamines from Alcohols and Primary Amines on a Nanoparticulated Gold Catalyst

A one-pot reaction was performed with a nanoparticulated gold catalyst. A secondary amine is formed through N-monoalkylation of a primary amine with an alcohol by a borrowing hydrogen methodology in a three-step reaction. The secondary amine formed enters into a second A(3)-coupling cycle to give propargylamines. The multistep reaction requires a gold species formed and stabilized on a ceria surface.

One‐Pot Palladium‐Catalyzed Borrowing Hydrogen Synthesis of Thioethers

Palladium on magnesium oxide is able to allow a one-pot reaction to synthesize thioethers from thiols and aldehydes formed in situ from the respective alcohol by means of a borrowing hydrogen method. The reaction is initiated by dehydrogenation of the alcohol to give a palladium hydride intermediate and an aldehyde. The latter reacts with a thiol involving most probably the intermediacy of a thionium ion RCH=S(+)R, which can be reduced in situ by the metal hydride to afford thioethers.

Photochemistry of nickel salen based complexes and relevance to catalysis

A series of four nickel salen complexes have been synthesised and their photochemical properties studied. These complexes emit in hexane at λem 580 nm with similar quantum efficiencies. Fluorescence decay occurs on the nanosecond time scale and its intensity is quenched by oxygen except for the complex with the shortest lifetime. For all complexes laser flash photolysis in CH2Cl2 or hexane generates a transient (lifetime on the μs time scale) that was assigned to a Ni-to-ligand electron transfer triplet excited state. In an attempt to show the relevance of photochemical data to catalysis, epoxidation of β-methylstyrene was carried out in the presence of the four complexes. It was found that the activity of the complexes correlates with the lifetime of the triplet charge-separated state. This relationship was interpreted as reflecting the intrinsic stability of the oxidised Ni ion in the different complexes, which is the common link between the photochemical and catalytic experiments.